FIX: Allow publish only when changes are pushed to `main` branch

[ppawlowski]

Description

Temporarily - run publish package job only when changes are pushed to main branch.

Related Issue(s)

Checklist

• [x] I have read the contribution guidelines
• [ ] Suitable unit/system level tests have been added and they pass
• [ ] Documentation has been updated
  • [ ] Upgrade instructions
  • [ ] Configuration details
  • [ ] Concepts
• [ ] Changes flowforge.yml?
  • [ ] Issue/PR raised on flowforge/helm to update ConfigMap Template
  • [ ] Issue/PR raised on flowforge/CloudProject to update values for Staging/Production

Labels

• [ ] Backport needed? -> add the backport label
• [ ] Includes a DB migration? -> add the area:migration label